Wavelength converting type radiant energy responsive display device



0d. 17, 1967 TABAC KOHASH, 3,348,056'

WAVELENGTi CONVERTING TYPE RADIANT ENERGY Filed May 2O' 1964 ESPONSIVEDISPLAY DEVICE 2 Sheets sheet l FIG. l @pagas l 7 ,Zhfe/-Zaye [am in e.cenf [mpeddn ce Ve777 r.l 72de@ Aww/; 2mm' WM f ATTORNEYS Oct. 17, 1967TADAO KoHAsl-u 3,348,056

- WAVELENGTH CONVERTING TYPE RADIANT ENERGY RESPONSIVE DISPLAY DEVICEFiled May 20. 1964 2 Sheets-Sheet 2 ATTORNEY Y P2M.

United i States Patent O 3,348,056 WAVELENGTH CONVERTING TYPE RADIANTENERGY RESPONSIVE DISPLAY DEVICE Tadao Kohashi, Yokohama, Japan,assignor to Matsushita Electric Industrial Co., Ltd., Osaka, Japan, acorporation of Japan Filed May 20, 1964, Ser. No. 368,906 Claimspriority, application Japan, May 22, 1963, 38/ 27,248 16 Claims. (Cl.Z50-213) ABSTRACT OF THE DISCLOSURE A multilayered radiant energyresponsive display device including a light transmitting base, a firstelectrode,

an electroluminescent layer, an interlayer, a photocon-V ductive layer,a third electrode, an impedance layer, a second electrode, and powersupply means. The impedance layer being an energy converting luminescentlayer having a radiant energy response to a first spectral distributionand having a luminescence characteristic With a second energydistribution different from the first and at least partly overlappingthe spectral photoconductivity distribution of the Vphotoconductivelayer whereby, when excited by radiant energy, the luminescent layerproduces luminescent light energy to which the photoconductive layer isresponsive,

trode including a wire, ribbon or reticular electroconduc-V tivestructure and interposedV between said electro-,-

luminescent and impedance layers in electrical contact with saidphotoconductive layer, electric power supply means for impressing avoltage V1 between said -first 4and third electrodes and another voltageV2 between said iirst and second electrodes, said voltages V1 and V2being variable or fixed at least in voltage value or polarity where theyare fed in DC form and at least in amplitude or phase where they are fedin AC form, and means for electrically controlling the luminescenceintensity of vsaid electroluminescent layer through the variation inimpedance under the excitation of the radiant energy on saidphotoconductive layer thereby to convert the radiant energy image intovisible form on said electroluminescent layer with or withoutamplification.

The radiant energy responsive display device has conventionally beencalled an electrostatic image converteramplifier. For discriminationbetween the inventive and conventional devices, however, only the latterwill conveniently be called an electrostatic image converteramplifierthroughout the following description.

The electrostatic image converter-amplifier employs a so-called secondelectrode and an impedance layer which are both transmissive to radiantenergy. With such device, it has been found and made known by theinventor that, by properly selecting or fixing the voltagesrVl and V2,the radiant energy image impinging upon the photoconductive layer can beconverted with or without ampliication into a visible image of thepositive, negative or Patented Oct. 17, 1967 ICC mixed nature on theelectroluminescent layer and that, by controlling the voltages V1 andV2, the contrast ratio, gamma and other operating characteristics of thedevice can be made variable over a wide range.

Moreover,with the positive image amplification characteristic, whichproduces an image of the positive nature, the radiant energy displayedcan be stored with a bistable luminescence intensity by feeding theluminescent rays from the electroluminescent layer back to thephotoconductive layer.

On the other hand, the photoconductive layer employed intheelectrostatic imageV converter-amplifier is vsubject to limitations dueto its material nature. For example, the spectral response distributionof the photoconductive layer is held lwithin considerably narrow limitsbecause ofV its properties deriving from the material of which the layeris formed and usually the layer is utterly non-responsive or onlyslightly responsive to some forms of radiant energy.

To cite an example, photoconductive materials of the type previouslyused most widely include cadmium sultide and cadmium selenide bothactivated with copper, chlorine, etc. and solid solutions of cadmiumsulfide and cadmium selenide. The spectral photoconductivitydistribution of this type of photoconductive material ranges from about500 ma to 900 mit, only partly covering the visible light range. Therange of distribution may be further limited under conditions ofpreparation for such material. Thus, the photoconductive layer formed ofsuch material is entirely non-responsive to light rays of the wavelengthof less than 500 m/L and also to ultraviolet rays. To the radiant energyof X-ray levels the material exhibits some sensitivity, which is verylimited when cornpared with the sensitivity to radiant energy in thevisible and near infrared regions. In addition, because of the highpenetrability, X-ray radiation is transmitted through thephotoconductive layer effecting only a limited amount of excitationthereto. This is the reason why the layer exhibits an extremely limitedsensitivity to X-ray radiation.

As for the electron radiation, for example, taking the form of a beam ofelectrons, it is blocked by the impedance layer, which customarily takesthe form of a solid layer. Thus, it has been impossible to excite thephotoconductive layer by electron the structure ofthe display device.

The present invention is intended to overcome these ditliculties.

According to the present invention, there is provided a radiant energyresponsive display device of the typev radiant energy responsiveluminescent impedance layer to effectively convert radiation energy intolight energy to which the photoconductive layer is most sensitive.

According to the present invention, it will be appreciated, therefore,that the display -device is operable with high sensitivity even toradiant energy 'of the level to which conventional electrostatic imageconverter-amplifiers have been inoperable or operable only with limitedrsensitivity because of the structure of the photoconduc- -tive layer.Moreover, the inventive device is extremely radiation because Vof simplein construction and highly eicient since it includes a radiant energyresponsive luminescent impedance layer which apparently serves also asan impedance layer essential to conventional electrostatic imageconverteramplifiers. Y

The foregoing and other objects and features of the present inventionwill become apparent from the following description when taken inconjunction with the accompanying drawings, which diagrammaticallyillustrate some embodiments of the invention. In the drawings:

FIG. 1 is a diagrammatic view of one embodiment of the present inventionincluding a luminescent impedance layer in the form of a single or mixedlayer, illustratingits structure and the power supply system therefor;

FIG. 2 is a diagrammatic illustration of the embodii' ment as used withradiant energy in the form of a beam of electrons;

FIG. 3 is a cross-sectional diagram of another embodiment of theinvention, which includes a luminescent impedance layer in the form of acomposite` layer,'illus trating its structure and the power supplysystem therefor; and Y FIG. 4 illustrates a .modification of theembodiment shown in FIG. 3, which further includes a DC control.

' shown on an enlarge scale. In the following description,

the radiant energy responsive luminescent impedance layer will bereferred to briefly as a luminescent impedance layer.

VFIG. l diagrammatically illustrates the structure of a radiant energyresponsive display device according to the present invention and .apower supply system therefor which are usable with radiant energy in theform of radiation such as X-rays, 'y-rays or an electron beam.

In FIG. 1, the reference numeral 1 indicates a lighttransmitting base orsupport plate, for example, formed of transparent glass sheet; andreference numeral 2 indi-l cates a rst electrode which islight-transmitting and formed, for example, of a metal oxide such as tinoxide.

The reference numeral 3 indicates a electrode-luminescent layer whichincludes a vapor-deposited film such as of ZnSV or a mixed layer formed,for example, of ZnS powder-activated with copper for green luminescenceandbound with glass enamel, plastic or the like material. Theelectroluminescent layer 3 is a solid layer having a thickness of theorder of from 5 toV 70,@ and emits light when excited electrically. Anopaque interlayer 4 is arranged so as to prevent any unstable operationof the device which may otherwise be caused by the excitation of aphotoconductive layer. 5 by the light feedback from the layer 3 orexterior light arriving from the base side of the device. The interlayer4 is a solid high-resistance layer of the thickness of the order of 1torlOfL, which includes an evaporated lm of an opaque high resistancematerial or is formed of a black paint, carbon black or the likematerial and a binder such as plastic or glass enamel. Thephotoconductive layer 5 includes a powder of photoconductive substancesuch as CdS activated with copper or chlorine and bound with plastic,glass enamel or the like binder material into layer form, or anevaporated ilm of such photoconductive substance including no bindingagent, or also a sintered film of such substance. The layer 5 has athickness of the order of from 5 to 100/r, and, containing CdS as anessential element, as described above, exhibits a high sensitivity tothe excitation of orange and infrared light energy with decreasingresistance. rIhe reference numeral 6 indicate a grid-like thirdelectrode, which in this embodiment is recticular. The grid-like thirdelectrode 6 includes a continuous metal structure like that of a metalnet electrode such as used in television pickup tubes, which includesconducting sections having a width of the order of, say 10 to 30u and athickness of the order of, say, several microns arranged in about 50 to100 mesh. Alternatively, the electrode 6 may 'be of a discontinuousmetal structure including tungsten or other metal wires of the thicknessof the order of, say,

l0 to 30p., weaved into a network of 50 to 100 mesh Vor n be plated withgold or rother metal. The third electrode 6k is arranged in electricalcontact with the photoconductiv layer 5. Y p l The reference numeralindicates a luminescent Yirnpedance layer which takes the form of asingle or mixed layer. For radiant energy E1 in `the `form of anelectron beam, the luminescent impedance layer 100 is formed of alluorescent material Such as (Zn, Mg)F2 for cathodoluminescence; and,for radiation energy E1, for example, in the form of X-rays or y-rays,the layer 100 is formed of a fluorescent material for'radiationluminescence such as ZnCdSzAg, which emits orange light energy to whichsaid photoconductive layer 5 is responsive withthe highest fsensitivity. Y

One form of such luminescent impedance layer -is' comprised of a mixedlayer formed of powdery luminescent or fluorescent material suchV asVdescribed above mixed with abinding agent such as epoxy resin or glass'Va single layer of uorescent material of the type described above formedby vapor depositing -or a single layer" formed by precipitating thefluorescent material preliminarily suspended in a solution of anappropriate cellulose such as nitrocellulose in aY suitable organicsolvent such as aluminum acetate and baking the layer of precipitate tovaporize the organic substances included therein while causing theprecipitate to cohere together to form a desired single layer, which isparticularly well adapted for use with electron beams.

The luminescent impendance layer formed in the manner described abovehas a high'resistivitypand exhibits a capacitive impendance to ACvoltages and whenrexcited with radiant rays or an electron beam emitsorgange light, to whichrthe photoconductive layer 5 is responsive withhigh sensitivity. The thickness of the luminescent imped-Y ance layer100 should be determined in due consideration of its relationship withthe impedance of the other layers and of its characteristics relative tothe radiant energy, including its transmissiw'ty, dielectric strengthand imped`` ance. For example, the Vthickness of the layer 100 isdetermined on the order of from 20 Vto 200g, depending upon thedimensions of the other layers cited hereinbefore.

The reference numeral 7 indicates a second electrode which istransmissive to radiation energy. For radiation E1 in the form of anelectron beam, the electrode 7 preferably includes a thin evaporated ilmof metal such as aluminum. For radiation E1 in the form of X-rays or thelike radiant rays of high penetrability, the electrode 7 iSV formed of asimilar evaporated film of aluminum or a thin lm of aluminum or the likemetal.

The second electrode 7 formed in the manner described above istransmissive to radiant energy and highly reflective to the luminescentlight energy as emitted from the layer 100 when the latter is excitedwith radiation energy so that the light energy is effectively preventedfrom escaping exteriorly through the electrode 7 and reflected towardthe layer 5. Thus, excitation of the photoconductive layer 5 can beeffected with high efciency by the luminescent light energy.

For radiations in the form of X-rays, fy-rays or like radiant rays, thesecond electrode 7 may include an electrically conductive coated layersuch as of silver paint.

VThe electrical power supply may be performed, for example, in themanner shown in FIG. 1. In the case where the electroluminescent layer 3is formed of a powder of electroluminescent material such as ZnS:Cu, Al,which emits green light when excited, and a binding agent such as epoxyresin, an AC power supply source is connected to the device since thelayer 3 is luminescent only when excited by an AC voltage. The thirdelectrode 6 includes a conductive strip 17 and an AC power source 8 isconnected between the strip 7 and the first electrode 2 by conductormeans 9 and 10 so that an AC voltage V1 is applied between electrodes 2and 6.

n the other hand, an AC power source 11 is connected between the 'irstand second electrodes 2 and 7 by way of conductors 9 and 12 to apply anAC voltage V2 of the same frequency as the voltage V1 between theelectrodes 2 and 7.

Under this condition, assume that V2=0 with the electrodes 2 and 7short-circuited through an external circuit and the frequency of voltageV1 is selected on the order of 100 to 5000 cycles per second at anappropriate voltage value. If the device is irradiated with an X-rayenergy image E1, the luminescent impedance layer 100 isexcited to emitlight which effectively excites the photoconductive layer 5, andsimultaneously the radiation E1, which is highly penetrable, pentratesthe layer 100 to excite the photoconductive layer 5. Thus, thephotoconductive layer is excited by two energy images, i.e. the X- rayenergy image and an orange light energy image, which has been formedfrom the X-radiation through conversion in wavelength, and to which thelayer 5 is most sensitive. As the result, the photoconductive layer Y 5is increased in conductivity in a plane at right angles to the directionof radiation E1, and a corresponding photocurrent is caused through thelayer 3 to electrically excite the electroluminescent powder thereinthereby to produce in the layer 3 a bright green image of the positivenature, which is amplified 'relative to the X-ray energy image E1 with asensitivity much higher-than that of anyY conventional electrostaticimage converter-amplifier.

Under this condition, if V1 is fixed and a dark state is establishedwith no radiation E1 given, a dark current' 1111 is formed inassociation with V1, which iiows between the third and iirst electrodes6 and 2 through the intermediary of the electrolurninescent layer 3.However, when voltage V2 is applied, a current I2 ilows between thefirst and second eelctrodes 7 and 2 by way of the luminescent impedancelayer 100, the interior of the rcticular structure of the thirdelectrode 6, and the-electroluminescent layer 3.

`Accordingly, if the voltage V1 is fixed and the phase difference of thevoltage V2 from V1 is selected so as t'o obtain a differential relationbetween the currents I1 and I2, the amount of amplitude of the currentI3=I1J|I2 iiowing through the layer 3 and hence the dark luminescencethereof are reduced as the amplitude of the voltage V2 increases. Suchphase difference is usually selected in the range of l80i90 dependingupon the impedance characteristic of the material employed. Withradiation E1 given under this condition, a visible image E2 of thepositive nature is obtained which has a contrast ratio and a gamma valueboth increasing with increase in amplitude of V2, rendering theoperating characteristics variable in a wide range. Contrariwise, if V1and V2 are both fixed in amplitude and the phase difference therebetweenmade variable, the contrast ratio and the gamma value can be controlledover a wide range in a decreasing direction as the phase relationshipdeviates from that for the above described differential relation betweenVI1 and I2.

Similarly, where V2 is applied, with V1=0, the current relation 13:12 isobtained and the layer 3 is effectively excited with the current I2.Under this condition, if radiation E1 is given, the photoconductivelayer 5 increases in conductivity in-accordance with the distributionpattern of the local strength of E1. It will be noted that, since theelectrodes 2 and 6 are at the same potential, the layer S behaves as anelectrostatic shielding layer and the cur- Y rent I2 is bypassed throughthe third electrode 6 to reduce the current 13:12, which is owingthrough the layer 3. In this manner, radiation E1 is converted, whilebeing intensified, into a bright visible image E2 of the negative naturerelative to radiation E1.

Under this condition with radiation E1, if the phase difference of V1from V2 is selected to obtain the difierential relation of I1(ordinarily in the Vrange of i90), the contrast ratio and the gammavalue are increased as the amplitude of V1 is increased from zero. Onthe other hand, as the phase difference is varied to deviate from thedifferential relation while fixing the amplitude of V1 and V2 at aproperly adjusted value, the contrast ratio and the gamma value arereduced accordingly. In the case of V1 has a more or less largeamplitude, the deviation of the phase difference causes reduction incontrast ratio and gamma value thereby to obtain a visible image E2 as amixture of a negative image corresponding to those portions of the imageE1 which areV limited in intensity and a positive image corresponding tothe high intensity portions of thev image'E1. Where the phase dierencebetween voltages V1 and V2 gives an additive relationship betweencurrents I1 and I2, an image E2 of the positive nature shows itselfwhich is entirely reverse to the one described above.

Assume that the amplitudes of V1 and V2 are selected so as to obtain anearly differential relationship between I1 and I2 and to render I3predominant over I2 when dark with no radiation E1 and over I1 when aradiation E1 of a uniform and high intensity is given.

Under this condition, if an X-ray energy image E1 of a suiciently highintensity is irradiated while maintaining I1 and I2 in a differentialrelation in whch I1 and I2 are in phase and of the same amplitude, theoutput image E2 obtained is extremely limited in brightness in areascorresponding to the localized intensity of the X-ray energy image E1and is of the negative nature in areas corresponding to those regions ofthe X-ray image having an intensity lower than said localized intensityand of the positive nature in areas corresponding to those regions ofthe intensity higher than said localized intensity. In this manner, theX-ray energy image is divided into two groups of intensity regions andis converted into an image of a combined negative and positive nature.In other words, the device exhibits a so-called V-shaped operationcharacteristic. In this case, if the amplitude of V1 is increased orthat of V2 decreased, the V-shaped characteristic is shifted in adirection in which the strength of the input X-ray radiation'increases,and similarly, if the amplitude of V1 is decreased or-that of V2increased, the V-shaped characteristic is shifted in a direction inwhich the X-ray radiation decreases in strength. Thus, the behavior ofthe output image E2 can be freely varied simply by controlling theamplitudes of V1 and V2, and precise observation and examination of theX-ray energy image E1 can be performed by a kind of zero method.

On the other hand,if, with the amplitudes of V1 and V2 xed, the phasedifference therebetween is caused to deviate from the ditierentialrelationship, the minimum Y luminescence intensity of the output imageE2 increases to reduce the contrast ratio while displacing the V-shapedcharacteristic. If the phase diiierence between V1 and V2 is varied toobtain an additive relation between I1 and I2, the output image E2varies continuously into an image of the positive nature.

As apparent from the foregoing description on several modes ofoperation, the inventive device is highly advantageous in that suchoperation can be effected continuously from one mode to another bycontrolling the amplitudes of the voltages V1 and V2 and the phaserelationship therebetween. Moreover, the device can be iixed to have aV-shaped characteristic of the positive, negative or intermediate naturesimply by properly fixing the voltages V1 and V2.

For the continuous controlling of the device or line adjustment thereofin a limited range, it is preferable to lemploy an AC power supplysource including a single signal generator and two electrical signalamplifier sys- Vtems for amplification of its AC signal output'with avariable phase-shifter and amplitude-controlling means arranged in atleast one of the amplifier systems. As forY a single power supply ifprovided with means to open the several electrodes of the device, toinsert an impedance element such as a resistor, an impedance or acapacitor or a combination of such elements between the electrodes orbetween the latter and the power source, or also to fix or make variablethe impedance value of such impedance element or combination ofimpedance elements. Further, la modification in which one power supplysystem includes an output transformer having vaV-tap changer or slidermeans on the output coil thereof lmay provide an entirely satisfactoryamplitude relationship between the voltages V1 and V2 and also asatisfactory phaseV relationship therebetween at least iu cases wherethe voltages are in the same or opposite phase.

Having described some forms of luminescent impedance layer 100, it is tobe noted that the dielectric strength of the layer 100 often-raises someproblem because of the required application of voltage V2. Such problem,however, can be alleviated by admixing a powder of dielectricsubstance'to the layer 100 which is light-reecting and has a highdielectric strength. For example, the luminescent impedance layer 100may be lformed by mixing a powder of uorescent material and a powder ofhighly light-reflecting dielectric substance such as zinc sulfide, zincoxide, titanium oxide or barium titanate and laminat- 'ing such mixturewith a binding agent such as epoxy resin. Apparently, it is necessary toproperly select a dielectric strength and an impedance value for thelayer 100.

On the other hand, the luminescent material is subject to spectrallimitations of the luminescent light energy and to other limitationssuch as the conversion efficiency of the material. Therefore, theselection of an impedance value resolves itself down to a matter ofcontrolling the thickness of the layer. On the other hand, highlylightreliecting materials provide a considerably wide range forselection of the dielectric constant. For example, barium titanatepowder has a dielectric constant of several thousands Vor over, andtitanium oxide of theV anatase form of the order of ten. Therefore, oneadvantage of the luminescent impedance layer 100 is that its impedancevalue can be selected in a wide range by properly selecting thematerials therefor and changing their volume ratio in the mixture. It isto be understood that the present invention also includes within itsscope such structures of luminescent impedance layer. Y

Though the foregoing description has been made with respect to theoperation on an AC power supply, the voltages V1 and V2 may also be ofDC form in cases where the electro-luminescent layer 3 takes the form ofan evaporated film such as of ZnS activated with Mn or other element,since such layer 3 is luminescent with a DC power supply. In such cases,various modes of operation as described hereinbefore can be obtained bycontrolling the voltage values and their polarity.

Also in these cases, it is obvious that the various cornponent layers ofthe device must be formed to have a proper conductivity to allow a moreor less DC current Y flow therethrough.

images, which are highly penetrable, an appropriate con-- ductivity canbe obtained without detracting fromtlie` utilizability of theluminescent light energy by admixing a powder of highly light-reectingmetal such as silver in a proper volume ratio.

Description will next be made on another embodiment of the presentinvention, which is shown in FIG. 2 and formed upon the basis of thesame principles as the one shown in FIG. 1.

The 'embodiment of FIG. 2 is basically similar to the radiant energyresponsive display device of FIG. 1, but isY particularly adapted foruse with radiationenergy in the form of a beam of electrons.

For convenience in description, the solid discrjgaortionV of the deviceis generally indicated at :200 and the power Y, source means forapplying voltages V1 and V2 at 300.L

The reference numeral 400 indicates an envelope similar to the picturetube used in a television set. The solid disc' f 200 being arranged in aportion corresponding to rthe uorescent screen infthe latter. Anelectron gun 401 emits a beam of electrons E1, which is modulated byanVelectrical signal S. Reference numeral 402 indicates an electron beamdeecting coil.

If, with voltages V1 and V2 applied, an electron beam E1 is directedontothe second electrode 7, which is comprised, for example, of anevaporated, aluminum film and is eletron-beam transmitting, the electronbeam penetrating through the second electrode 7 excites the luminescentimpedance layer 100, which includes at least aL cathode luminescencematerial and when excited emits light energy, which excites thephotoconductive layer 5. In this manner, it will be understood that theluminescence outputEZ of the electroluminescent layer 3 can becontrolled electrically.

For example, if a video signal is used as electrical signal S to controlelectron beam E1, it can be converted into a visible image image, whichappears on the first electrode (2) side of the assembly under control ofthe dellectingV coil 402.

This embodiment is thus operable in the same manner as conventionaltelevision picture tubes and, exhibiting a` Y much higher amplificationfactor, can operate satis-l y fa-ctorily with an electron beam E1 lowerin voltage .and current and with a video signal S weaker than anyconventional television picture tube. Infaddition, the output imageobtained with the device is brighter because of its higheramplification.

An extremely important advantage of this device is that not only thecontrast ratio and the gamma value of the image can be freely controlledby varying voltagesV V1 and V2.

A further embodiment of the present invention shown in FIG. 3 isparticularly adapted for use with a radiation energy image E1 such as anultraviolet image, which is relatively low in penetrability.

A major feature of this embodiment is that its luminescent impedancelayer is of composite form.

Description willnow be made assuming that E1 is an ultraviolet image. InFIG. 3, reference numeral 101'indicates a luminescent layer at leastcontaining photoluminescent material such as CdZnS:Ag, which emitsorange light when excited with ultraviolet image E1. The layer 101 canbe formed in the same manner as described in connection with FIG'. l.When excited, the layer 101 can luminescense `only in its surface regionsince the ultraviolet image E cannot fully excite the interior of thelayer because of its limited penetrability. 1

Therefore, the excitation of photoconductive layer 5 is effected by theluminescense light energy coming fromV the surface region of the layer101 therethrough and such excitation may not be satisfactorily effectivebecause the light energyY is partly lost by absorption during its 9passage through the material of the layer 101. To avoid this, the layer101 itself should be made extremely thin.

On the other hand, the thickness of the layer l101 is subject tolimitations from the impedance and dielectric strength requirements tothe layer 100 as a luminescent impedance layer.

This situation can be improved by employing an auxiliary luminescencelight energy transmitting impedance layer 102 between the luminescentlayer 101 and photoconductive layer 5. The impedance layer 102 is formedof a transparent low-loss dielectric substance, such as a polyester lmor a light-transmitting glass enamel. With this construction, theproblems of the impedance and dielectric strength can be met with thelayer 102, allowing the layer 101 to have any desired thickness. Thelight energy produced in the layer 101 when excited by ultraviolet imageE1 passes through the layer '102 to eiectively excite thephotoconductive layer 5,V as will readily be understood. v

By use of the composite luminescent impedance layer 100 includingelementary Vlayers 101 and 102, it will be appreciated that anyultraviolet image E1 can be converted or amplified into visible imagesE2 of dijerent natures by controlling the voltages V1 and V2, asdescribed hereinbefore in connection with FIG. 1, though such imageconversion is impossible with photoconductive layer 5 formed ofphotoconductive material such as CdS:Cu, Cl.

In this embodiment, the second electrode 7, which is radiant energytransmitting, is comprised of a base or support plate, such as a quartzplate 14, which is transmissive to radiant energy and,.in this instance,to ultraviolet rays, and an electrically conductive lm of tin oxide orthe like material coated on said base plate 14.

The third or grid electrode 6,1'nthis embodiment includes an arrangementof spaced parallel metal wires and a conducting strip 17, between whichvoltage Vlis applied from power supply source 8, as illustrated.

The embodiment of FIG. 3 also includes another form of interlayer 13lying between the opaque layer 4 and electroluminescent layer 3. Thelayer 13 is designed to prevent dielectric breakdown between theelectrodes 2 and 6 and also to reliect the luminescence light from layer3 thereby to enhance the brightness of the visible output image E2. Thelayer 13 may be formed by evaporating a high dielectric white substancehaving a high reflection factor and a high dielectric strength, forexample, titanium oxide or barium titanate, or by laminating a mixtureof a powder of such substance and a binding agent such as glass enamelor plastic.

The thickness of the interlayer 13 is determined so as to give animpedance lower than that of layer'3 with the intention of reducing thevoltage loss.

The wire grid electrode 6, serving as a discharge electrode, includestungsten or other metal wires of the thickness of the order of to 150;arranged at regular intervals of the order of 250 to 700i; and, ifrequired, plated with gold.

In this example, the grid electrode 6 is formed of thin tungsten Wiresof the thickness of about 10u and is embedded in the relatively-thicklayer 5 midway of its thickness of about 80p. Y

It will be understood that this embodiment is also usable on the sameprinciples with radiation energy El taking the form of X-rays, 'y-raysor like radiant rays of high penetrability. With electron beams,satisfactory results can be obtained by eliminating base plate 14 andmaking electrode 7 in the form transmissive to electron beams, forexample, in the form of a evaporated aluminum film.

FIG. 4 illustrates another embodiment of the present invention which isparticularly useful with radiation energy forms of high penetrabilitysuch as X-rays and fyrays. In this embodiment, the luminescent impedancelayer 100 is also a `composite layer, but of the construction differentfrom that in the embodiment of FIG. 3.

Description will now be made on the embodiment as used with a radiationenergy E1 in the form of an X-ray image.

In FIG. 3, the photoconductive layer is excited by luminescent lightrays which have passed through the impedance layer 102, which istransmissive to luminescent light energy. In that case, the light raysare often dispersed in the layer 102 to such an extent as to make outputimage E2 hazy.

This diiiculty is overcome in the embodiment of FIG. 4 by layingdirectly on thesurface of the photoconductive layer 5 a Vluminescentlayer 101 which at least contains a radiation luminescence material asone mentioned hereinbefore. A radiant energy transmittingauxiliary'impedance layer |103 is provided between the second electrode7 and layer 101, as illustrated, for the purpose of improving thedielectric strength and impedanceV characteristics of the assembly. Theauxiliary impedance layer 103 in the illustrated example is formed of anX-ray transmitting dielectric substance such as polyester or otherplastic film, glass film or glass enamel.

The second electrode 7 is an X-ray transmitting electrode which islight-reilecting With an aluminum or other metal lilm evaporated or analuminum or other thin metal foil stuck thereon. Also, utilization maybe made of an electrically conducting ilm of a metal oxide such as tinoxide coated on a thin base Iplate of glass.

In addition, an appropriate light reflectivity may be imparted to thelayer 103 ,to enhance the utilization factor of luminescent light energyfrom the layer 101. In other Words, the layer 103 may be made radiationenergy transmissive and luminescent light energy reflecting to serve asan auxiliary impedance layer. For example, for use with X-ray images,the layer 103 should leastwise contain a highly light-reilectingsubstance. One example is a single layer of magnesium oxide or likematerial coated on the luminescent layer 101. Another example takes theform of a mixed layer including a fine powder of zinc sulfide, titaniumoxide, barium titanate or like compound mixed with a binding agent suchas epoxy resin orV other plastic or glass enamel. I

Withthese forms of composite layer 103, the luminescent light energyfrom layer 101 is subjected only to a minimized light-dispersing effectand can be utilized in the photoconductive layer 5 etliciently enough toform a clear and definite output image E2, since it is directlyreflected by the layer 103 toward the photoconductive layer 5. Theseforms of composite layer 103 are obviously highly transmissive to X-rayssince they are usually made considerably thin.

In the case where the layer 103.is transmissive to luminescent lightenergy and the latter is reilected by the second electrode 7, asdescribed hereinbefore, the light energy must proceed over a substantialdistance and is subject to a considerably high light-dispersing effectin the layer 103. This sometimes causes an unnegligible detraction fromthe quality of the output image E2.

If the layer 103 is made opaque or light absorptive in an attempt toeliminate such adverse eiect, it will absorb the luminescent lightraysand the utilization factor of the luminescent light energy and hencethe sensitivity of the device will be reduced correspondingly. y

It follows, therefore, that to impart a light reectivity to the layer103 is a very effective way to save this difliculty.

The use of impedance layer 103 is also advantageous when viewed from thestandpoint of controlling its impedance value in that an appropriateimpedance value can readily be selected for the layer while improvingthe dielectric strength thereof by properly selecting the thickness ofthe layer and, if it is a mixed layer, the specific dielectricity andvolume ratio of the powder materials forming the layer.

The power supply system shown in FIG. 4 is particularly well adapted tocontrol the operation characteristics of the inventive display devicedirect-currentwise over a 4 widely extended range While improving'thesensitivity thereof, incase the device includes a photoconductive layerin the .form of a mixed layer including photoconductive powder materialsuch as CdS:Cu, Cl and electrodes, which are connected with a variablecurrent source 16 by way of a polarity changing switch 15 and with an ACpower source 300 by way ofdirect-current blocking capacitors C1 and C2and a conductor 10. Capacitors C1 and C2 should have a capacitancesuicient to minimize their AC impedance. Y

As illustrated, the AC power source 300 applies an AC voltage V1 betweenconductors 9 and 10 and another AC voltage V1 between coductors 9 and12. Under this situation, AC voltage V1 is applied between the-thirdelectrode 3 and light-transmitting electrode 2, and DC voltage VBbetween the two sets of alternate elements in the third electrode -6 (inthe plane of the photoconductive layer 5 extending at right angles tothe direction in which the X-ray energy El is irradiated). 4

The AC photoconductive sensitivity of the photoconductive layer, whichis formed of a mxiture of photoconductive powder such as CdSzCu, Cl Witha .binding agent, as described hereinbefore, is reduced as the.frequency increases because of the nonlinearity of its voltage-currentcharacteristics and inherent AC dependence. This reduction in ACphotocoductive sensitivity can be ameliorated -by use of a DCcontrolling voltage. Y'

On the other hand, the sensitivity, operation characteristics Vcontrastratio and gamma value of the radiant energy responsive display deviceshown in FIG. 4 are dependent upon the photoconductive sensitivity in aplane normal to the direction in which E1 is irradiated. Therefore,these characteristics of the device can be made variable by use of DCvoltage VB from the variable DC' power source 16 to controllablyincrease the AC photoconductive sensitivity in a plane normal to thedirection of radiation E1. The range of variation in the performancecharacteristics'will be extended as the voltages V1 and V2 increase infrequency.

In the positive, V-shaped and negative operations of the device asobtained by controlling V1 andKV2 in the same manner as described inconnection with FIG. 1, the increase in VB causes displacement of theoperation characteristics in a direction in which the irradiationmagnitude of the input radiant energy decreases thereby to increase thesensitivity, contrast ratio and gamma value. An irnportant advantage isthus obtained that the range of variation in performance of the devicewhich is obtainable by controlling the amplitude and phase relationshipsof V1 and V2 (including the case where either V1 or V2 is reduced tozero)can be further extended under the control of DC voltage VB.

On the other hand, use of a photoconductive layer formed of powderymaterial bound together is very undesirable fon some applications sinceeven after the irradiation of the radiant energy image has beeninterrupted the speed ofresponse is so limited as to cause a residualimage continuing for a period of from several seconds to severalminutes.

Utilizing a special phenomenon of this form of photoconductive layer,the residual image can be extinguished rapidly by changing the polarityof the D C. voltage VB, which is being applied between the interlinedelectrodes 6, thereby to reverse the polarity of the D.C. electric eldin which the photoconductive layer 5 is placed.

The polarity-changing switch 15 is provided to serve the' purpose. Withthe power supply system shown in'FIG. `4, it will thus be appreciatedthat not only the performance characteristics of the display device canbe made variable over a wider range -but also the residual image can befreely extinguished.

This power supply system can be applied irrespective of the structureofV the third electrode 6 as long as it includes an arrangement ofelectrode elements, for example, as shown in FIG. 3, and without regardto the type of radiant energy image El.

Obviously, voltage means 16 is not always required to be variable sincethe D C. voltage may be fixed if required.

Though the luminescent impedance layer 100 has been shown and describedherein as including one'or two layers,

it may take the form of a composite layer including moreA withoutldeparting from the than two elementary layersV spirit of the invention.A

` It will be appreciated Yfrom'lthe foregoing that the.

- present invention makes it possible to convert a radiant energy imageVinto a visible image vintensified with highY` Y sensitivity, which'haspreviously been impossible or feas- Y.

ibly only with limited sensitivity.

The radiant energy responsive display-device oftheY`v`r presentinvention is usable withva wide variety of radiant Yenergy images:including those having wavelengthsshorter than the highest sensitivitywavelength in the` spectrum or conductivity distribution of thephotoconductive layer, Y

whether they are visible or invisible` (like ultraviolet rays);

radiation energy images such as X-ray or y-ray radiation' l images;Velectron radiation images or signals such as electron beams; and allother 'radiant energy signals or 1 images elective to excite theluminescent impedancelayer of the device. Particularly, with highpenetrability radiation energy images such as X-ray or 'y-ray images,the` inventive device can operate with such a high sensitivity as haspreviously been unimaginable since the photoconductive layer is excitedwith two types of image including the radiant energy image passingthrough the luminescentV impedance layer and a luminescent light energyimage formed under the excitation elfect of the radiant energy- Y image.

Though a few embodiments of the present invention Y have been shown anddescribed herein, it is to understood that many changesV andmodifications can be made by combining the features and concepts'of thedifferent Y embodiments without departing from the spirit of the in-Vvention or from the scope of the appended claims.

What is claimed is:

1. A radiant energy responsive display device of the type including aphotoconductive layer, an electroluminescent layer lying on one surfaceof said photoconductive layer, an impedance layer lying on the othersurface of said photoconductivelayer, a first electrode arranged on theouter surface of said electroluminescent layer, Va second electrodearranged on 'the outer surface ofV said impedance layer, a thirdelectrodeinterposed between said electroluminescent and impedance layerslin electricalv l' contact with said photo'conductive layer, electricpower supply means for'applying a Iirst voltage between said rst andthird electrodes and second voltage between said first and secondelectrodes, an means for electrically controlling the luminescentintensity of said electrolumil `nescent layer through the variation inimpedance under the excitation effect of radiant energy on saidphotoconductive layer to thereby convert'V a radiant energy image 2 intovisible form on said electroluminescent layer, in which said impedancelayer is an energy converting luminescent layer having a radiant energyresponse to` a first spectral distributionV and having a luminescencecharacteristic with a second energy distribution different i from therst and at least partly overlapping the Y spectral photoconductivitydistribution of the photoconductive layer whereby, when excited withradiant energy,

said luminescent impedance layer produces luminescent light energy towhich said photocondnctive layer is responsive. Y

2. A radiant energy responsive display device according to claim 1wherein an intermediate layer is interposed between saidelectroluminescent layer and said photoconductive layer.

3. A radiant energy responsive display device according to claim 1wherein said intermediate layer is an opaque layer.

4. A radiant energy responsive display device according to claim 1wherein said rst electrode is a light transmitting electrode.

5. A radiant energy responsive display device according to claim 1wherein said second electrode is a radiant energy transmittingelectrode.

6. A radiant energy responsive display device according to claim 1wherein said third electrode is a grid-like discharge electrode.

7. A radiant energy responsive display device according to claim 1wherein said radiant energy image is converted to visible form on saidelectroluminescent layer at a size no less than that of the originalimage.

8. A radiant energy responsive display device according to claim 1 inwhich the maximum spectral sensitivity of said photoconductive layerlies within the spectral range of the luminescence of said luminescentimpedance layer.

9. A radiant energy responsive display device according to claim 1 inwhich said luminescent impedance layer has a thickness of 20 to 20D/i.

10. A radiant energy responsive display device according to claim 1 inwhich said impedance layer comprises a compound of zinc and epoxy resin.

11. A radiant energy responsive display device according to claim 1 inwhich said impedance layer comprises a compound of zinc and glassenamel. t

12. A radiant energy responsive display device according to claim 1 inwhich said impedance layer comprises a highly light reflective material.

13. A radiant energy responsive display device according to claim 1 inwhich said impedance layer comprises a mixture of powder of aluminescent material and a binding material.

14. A radiant energy responsive display device according to claim 1comprising an auxiliary impedance layer capable of transmitting saidluminescent light energy produced by said impedance layer interposedbetween said impedance layer and said photoconductive layer.

15. A radiant energy responsive display device according to claim 1comprising an auxiliary impedance layer capable of transmitting saidradiant energy interposed between said impedance layer and said secondelectrode.

16. A radiant energy responsive display device according to claim 1comprising an auxiliary impedance layer capable of transmitting saidradiant energy and capable of reflecting lsaid luminescent light energyproduced by said impedance layer interposed between said impedance layerand said second electrode.

References Cited UNITED STATES PATENTS 2,929,950 3/ 1960 Hanlet 313-1082,999,941 9/ 1961 Klasens et al Z50-213 3,101,408 8/1963 Taylor 250-83.3X 3,217,168 11/ 1965 Kohashi 250--213 WALTER STOLWEIN, Primary Examiner.

RALPH G. NILSON, Examiner.

I. D. WALL, Assistant Examiner.

1. A RADIANT ENERGY RESPONSIVE DISPLAY DEVICE OF THE TYPE INCLUDING APHOTOCONDUCTIVE LAYER, AN ELECTROLUMINESCENT LAYER LYING ON ONE SURFACEOF SAID PHOTOCONDUCTIVE LAYER, AN IMPEDANCE LAYER LYING ON THE OTHERSURFACE OF SAID PHOTOCONDUCTIVE LAYER, A FIRST ELECTRODE ARRANGED ON THEOUTER SURFACE OF SAID ELECTROLUMINESCENT LAYER, A SECOND ELECTRODEARRANGED ON THE OUTER SURFACE OF SAID IMPEDANCE LAYER, A THIRD ELECRODEINTERPOSED BETWEEN SAID ELECTROLUMINESCENT AND IMPEDANCE LAYERS INELECTRICAL CONTACT WITH SAID PHOTOCONDUCTIVE LAYER, ELECTRIC POWERSUPPLY MEANS FOR APPLYING A FIRST VOLTAGE BETWEEN SAID FIRST AND THIRDELECTRODES AND SECOND VOLTAGE BETWEEN SAID FIRST AND SECOND ELECTRODES,AN MEANS FOR ELECTRICALLY CONTROLLING THE LUMINESCENT INTENSITY OF SAIDELECTROLUMINESCENT LAYER THROUGH THE VARIATION IN IMPEDANCE UNDER THEEXCITATION EFFECT OF RADIANT ENERGY ON SAID PHOTOCON-